panel compatibility question

I have a 24 volt off grid system with 4 12 volt BP125 watt panels. Two panels are tied together then go to the combiner box. I was told by my installer that if I wanted to add 24 volt panels I would have to wire them to a separate combiner box and charge controller to do that. My question is could I not just wire each 24 volt panel to the combiner box? I would love to add the higher watt panels that are available in 24 volt vs what I can get in 12volt. Thanks for your help.
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Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question

    What "24 volt panels" are you thinking of adding? True 24 volt panels (Vmp~35-40 volts Vmp) are kind of rare these days... Many panels are Vmp~30 volts which is too low of voltage to properly charge a 24 volt battery bank.

    You would need to put two Vmp~30 volt panels in series and get a MPPT type charge controller to properly/efficiently use these panels on a battery bank.

    What brand/model of charge controller do you have today?

    And the combiner box you presently have--How many breakers/fuses does it have? With 3 ore more parallel strings of panels, you need a fuse/breaker per parallel connection for electrical safety.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question

    I dont have a specific panel chose yet but like that I could get 250 or even 300 watt panels in 24 volt vs 150 watt is as good as I have found in 12volt. Mainly just been looking on ebay. I guess its a bang for the buck thing for me. I have a pole mount rack that has my 4 125 watt panels currently and have room for 6 more panels. I would like to get as many watts up as I can. My charge controller is a 50 amp Solar Boost and outback inverter. I can get model number, system is at my cabin. I know that if I add more panels I will have to upgrade to a 80 or 100 amp controller. My system was professionally installed in 2004. I had 8 interstate HL 16 batteries that I just swapped out for 8 Full River DC400 batteries. So I am way underpowered on charging for the batteries I have. As to my combiner box I will get down there and see for sure but it should have capacity for 10 panels as that was the plan when I had it installed originally.
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question

    Most of the "GT" panels these days (best $$$/Watt pricing) over ~140 Watts have Vmp~30 volts.

    To properly charge a 24 volt battery you would (with typical "high end" MPPT charge controller) put two to three panels in series for Vmp-array~60/90 VDC.

    Your Solar Boost 50 is probably not going to work with such a high Vmp-array voltage--Just leave the existing panels+SB50 connected as is, and get a second MPPT charge controller for the new array. Parallel the new charge controller with the existing SB50.

    Also, make sure you have the battery bank wired "correctly" so that the batteries will properly share charging/discharging current.

    If I understand your battery configuration, you have an 800 AH @ 24 volt battery bank. Using the 5% to 13% rule of thumb for charging, your solar array should be around:
    • 800 AH * 28.4 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,475 Watt minimum array
    • 800 AH * 28.4 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,951 Watt array nominal
    • 800 AH * 28.4 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,836 Watt array "cost effective" maximum
    For a full time off grid installation, 5% rate of charge is the minimum we would recommend... 10%-13% is a nice healthy array size. (the above numbers are so you can reproduce my math--For solar, any numbers within ~+10% of the calculated numbers is usually pretty close to "the same").

    With the low cost of solar panels and the high costs of battery banks, it usually makes a lot of sense to put 10%+ worth of panels on your battery bank--Unless this is just a weekend/seasonal/emergency backup system.

    An 800 AH battery bank--You are looking around 80+/- amps of charging current--No small amount of current/energy. Depending on your loads and such--You are about the point where you may wish to look at a 48 volt battery bank (for example, if you need to replace your AC inverter). That would cut the charging current to 1/2 and allow you more charge controller options/larger solar array support.

    You got a good life out of your first set of batteries--So I will not go too far down the battery management/maintenance road for now--Just make sure your new charge controller is setup for AGM and you do not ever perform "high voltage" equalization. AGMs are "easy" to kill with over charging (uses up internal catalyst, can cause batteries to vent irreplaceable electrolyte).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question

    Thank you for all the information. Yes my batteries are wired in two strings of four. I bought my new batteries from the guy that installed my system and he said exactly what you did about differences in charging. Only had these batteries for about a month and if anything I will have to adjust my charging up as controller is kicking into float mode at around 27. My lead acid batteries were set up for 29.2. Have not changed any settings so I am not sure why the charge controller is kicking into acceptance and float at lower voltages unless shutting system down when I put new batteries in reset the charge controller to default settings. The Full River agms actually spec at about the same as my old batteries as far as charge but I was planning to err on the side of caution and not go over 28.6-28.8 tops.

    Thanks for the numbers on how much panel I need to properly charge my battery bank. If I fill out my current array with the biggest 12 volt panels I can get(150 watt) I will still come up just short of the minimum needed to maintain my battery bank. Though I am not there full time so I think would be ok for occasional use. If my current charge controller will not handle the larger panels then it makes the most sense to just fill out my current array with basically what I am using now and then down the road set up a new array next to it with larger panels and separate charge controller. Am I on the right track with my thinking?
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: panel compatibility question

    Do you have a generator to do BULK charging and solar to finish off the Absorb and Float?
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question
    mark68 wrote: »
    Thanks for the numbers on how much panel I need to properly charge my battery bank. If I fill out my current array with the biggest 12 volt panels I can get(150 watt) I will still come up just short of the minimum needed to maintain my battery bank. Though I am not there full time so I think would be ok for occasional use. If my current charge controller will not handle the larger panels then it makes the most sense to just fill out my current array with basically what I am using now and then down the road set up a new array next to it with larger panels and separate charge controller. Am I on the right track with my thinking?
    Sounds OK... You should make several paper designs and see what works out best for you.

    The larger wattage panels (>140 watts) tend to be much cheaper--But a high end MPPT charge controller is more expensive. Usually no "cheap" short cuts with solar.

    ~140 Watt panels right now are around $2+ per watt... 190 Watt and above panels are around $1 per Watt. I see our host has a nice true "190 Watt 24 volt / 72 cell" panel (Vmp~36 volts) for near $1 per Watt:



    Eoplly Solar EP125M-72-190
    $218.50
    $1.15




    But--Be careful that you get full cost delivered to your door (shipping and insurance). Panels over ~140 watts usually ship by truck. And less than full pallet loads of panels need repackaging and expensive shipping (a single panel can almost double in price once shipping is included).

    Also, panels over ~175 watts usually need two people to safely install them on a roof/rack (physically large/heavy panels).

    You might see if there are any solar installers in your neighborhood and see what their panels cost out the door. Usually these will be GT Designed panels with Vmp not ~17.5/36 volts--And they would need an MPPT type charge controller (probably different than the SB50).

    Keep an eye on the charge controller. When they start acting a bit strange--There could be connection problems (dirty/loose/corroded) with the battery bank or internal controller failure. I would suggest a DC Current Clamp DMM like this inexpensive one from Sears to make it easier to monitor/maintain your system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question
    westbranch wrote: »
    Do you have a generator to do BULK charging and solar to finish off the Absorb and Float?

    I have a onan 6500 watt generator. I lived there full time in 2004-5 but am only there a few times a month now.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: panel compatibility question

    That being then case you could use the gen for Bulk before you leave on the last day and let the PV finish off the charge.
    OR
    look at one of the new smaller MPPT chargers that is out there, Midnite KID, Rogue, and FULLY build out a second bank to add to bank 1.
    You will have to run the numbers to see what works for you.

    PS you did not tell us which Outback Inverter (Inver/charger?) you have
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question

    I would love a second bank but I think what I have now is good. I could maybe see one more string of 4 to add to what I have. It is just 3 of us on weekends now and full time when we retire. I have a Bosch tank less water heater,wood stove and vent less propane wall heaters for backup heat. Its more of an off grid cabin. All my lights are the low watt florescent bulbs. Stove is propane. Fridge is 9 cu ft magic chef that draws 4 amps. We have all the normal house items like washer,microwave,coffee maker. We do laundry during peak hours, coffee goes into a thermos when made. TV,vcr,dvd player. I am way short on charging capacity for the batteries I have. I will get the model of outback inverter I have.My installer has all that on file.I seem to have misplaced my information after a few years and a few moves.

    I never had any charging issues with my lead acid batteries. Usually if we got there during a sunny day they would either be in acceptance or float mode and if it was cloudy they would be in bulk charge mode but had a healthy state of charge. I think my issues now with my new batteries is either a connection issue with the charge controller or my controller reset to default settings when I shut down the system to change out my batteries.
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question

    Thanks BB. I think I will fill out my current array with 12 volt panels. I only would have gone the larger panel route if they were compatible with my current equipment.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: panel compatibility question

    sorry , I have to be a bit blunt here: What you have in PV right now is a way to kill your batteries over the long haul. You are deficit charging the minute you turn on your charger and don't use the gen set the next day.

    Using Bill's calcs you have LESS THAN 1/3 of the minimum Watts that can keep your batteries charged. If the batteries are fully charged when you leave they should be be maintained by the PV's as long as the inverter is shut down.

    The problem with deficit charging is the development of sulphation on the plates, the longer the process goes on the less chance you have of recovering the cells with an EQ or other charging plan like longer Absorbs, higher voltage settings, etc.

    hth
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question
    westbranch wrote: »
    sorry , I have to be a bit blunt here: What you have in PV right now is a way to kill your batteries over the long haul. You are deficit charging the minute you turn on your charger and don't use the gen set the next day.

    Using Bill's calcs you have LESS THAN 1/3 of the minimum Watts that can keep your batteries charged. If the batteries are fully charged when you leave they should be be maintained by the PV's as long as the inverter is shut down.

    The problem with deficit charging is the development of sulphation on the plates, the longer the process goes on the less chance you have of recovering the cells with an EQ or other charging plan like longer Absorbs, higher voltage settings, etc.

    hth

    Hey no apologies necessary. Thanks for the heads up! I had nothing powered up until last week. When I left to come home I left the fridge on to "work" the batteries. I will shut it down again when I get down there next weekend. And I wont leave anything "powered up" until I have sufficient PV charging capability installed. Once again thanks for the honesty.
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question

    Be really careful. A system left on and unattended (AC inverter + Fridge, or DC Fridge) on your, very expensive battery bank, with no low voltage/low State of Charge cutoff--Is very risky.

    Taking deep cycle batteries below ~50% can shorten their life. Taking below 20% can kill them.

    You only need about 1% rate of charge to "float" an already charged (>~90% SOC) battery bank.

    And "things happen" (breaker pops, charge controller "crashes", etc.)--Unattended loads can really ruin your day.

    And designing/installing a "fail safe" backup can be expensive and complex (the redundancy problem... The more complex you make your install for redundancy, the more things that can fail, needing more redundancy, leading to more things failing)...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: panel compatibility question

    First, this is the first time I have heard of these new AGM batteries.....and I don't recall anyone else referencing them either so comments are just that.

    You are right that on most FLA batteries there is a break in period, but I have not seen a reference to that for AGM's, so it may or may not be needed. Did the seller mention it?
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question
    BB. wrote: »
    Be really careful. A system left on and unattended (AC inverter + Fridge, or DC Fridge) on your, very expensive battery bank, with no low voltage/low State of Charge cutoff--Is very risky.

    Taking deep cycle batteries below ~50% can shorten their life. Taking below 20% can kill them.

    You only need about 1% rate of charge to "float" an already charged (>~90% SOC) battery bank.

    And "things happen" (breaker pops, charge controller "crashes", etc.--Unattended loads can really ruin your day.

    And designing/installing a "fail safe" backup can be expensive and complex (the redundancy problem... The more complex you make your install for redundancy, the more things that can fail, needing more redundancy, leading to more things failing)...

    -Bill
    Thanks Bill. If I was to shut down my system when I leave and have no draw on the batteries am I shortening their life? That was the main reason for leaving the system on with the lead acid batteries. Is it less of an issue with agm batteries?
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: panel compatibility question

    I just finished doing a quick review of these documents, http://www.magnacharge.com/index.php?option=com_content&view=article&id=77&Itemid=240
    and did not see any reference to conditioning those AGMs. AGMs are a very different bird than FLA's.

    Interesting that they are different in how to install the cells, all the ones I have say to orient the cells horizontally to ensure proper distribution of the sulphuric acid through the glass mat (of AGM)
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question
    westbranch wrote: »
    First, this is the first time I have heard of these new AGM batteries.....and I don't recall anyone else referencing them either so comments are just that.

    You are right that on most FLA batteries there is a break in period, but I have not seen a reference to that for AGM's, so it may or may not be needed. Did the seller mention it?

    No he did not say to do anything special but the reason I did not leave anything on until last weekend was it was my first chance to spend the weekend down there with everything powered up to see how the batteries charge during the day and discharge at night. From what I have read and been told,AGM batteries are maintenance free like gel cells (good as I am not there every day) and the charge characteristics closer to a lead acid battery. The specs for the AGM batteries I have actually have almost identical acceptance and float settings as the lead acid batteries that I had.
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question

    As long as the charge controller has the correct float voltage and all (unneeded) loads are turned off/disconnected--You should be fine.

    Discharging a battery bank to 75% State of Charge (1/4 discharge) once a month is usually helpful (or at least a 10% discharge minimum).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: panel compatibility question

    According to the chart here
    http://www.fullriverdcbattery.com/technical/charging/

    the FLOAT should be at 2.8A or less . ABSORB at 29.4V for 8 Hrs, which is quite long. Almost like NiFe batteries IIRC.

    I recommend you do some 'light' reading on the nature of AGMs and how they react to various charge levels..

    Try this one, http://www.cdtechno.com/pdf/ref/41_2128_0212.pdf ( Charging Regulated VRLA Batteries) top of the VRLA grouping
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question
    westbranch wrote: »
    I just finished doing a quick review of these documents, http://www.magnacharge.com/index.php?option=com_content&view=article&id=77&Itemid=240
    and did not see any reference to conditioning those AGMs. AGMs are a very different bird than FLA's.

    Interesting that they are different in how to install the cells, all the ones I have say to orient the cells horizontally to ensure proper distribution of the sulphuric acid through the glass mat (of AGM)
    Thanks for the link. I saw on page 11 where it said AGM batteries can sit unused for a year or more unlike lead acid. Which I would never go that long. So I will shut down my system every time I leave from now on. Do you guys think I would be extending the life of the batteries by not cycling my batteries unless I am there using my system? I just saw that you have agm as well. Are you happy with them as opposed to lead acid?
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question

    Probably 6 months maximum is a safer storage time between charging for AGM.

    It all depends on self discharge rate. AGMs when new have very low self discharge. As they age (and if stored warm/hot climate), self discharge rate goes up.

    An "about to fail" battery bank will get upwards of 2% self discharge rate (i.e., over heating, exploding, etc.) from one company (that maintains AGM/Lead Acid Battery banks for large companies).

    Otherwise, a good AGM will have a few percent per month self discharge rate--And usually ~75% State of Charge seems to be the "cutoff" point (where sulfation becomes a larger issue).

    Placing your batteries on "float" with a solar charger (not too high of float voltage)--You should be fine. If you are coming back every few months and charging--Then letting the AGMs sit disconnected is not bad either.

    -Bill "at least what I have read" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: panel compatibility question
    mark68 wrote: »
    Do you guys think I would be extending the life of the batteries by not cycling my batteries unless I am there using my system?

    Yes, a battery only has so many cycles in its life. Disconnecting them in your absence makes sense, but they should be fully charged when you leave. That may be a problem... are you going to run the generator so that they can get the 8 hours of absorb they need?

    With an undersized array, there is something to be said for leaving them connected to the controller while you are gone... they will get charged over the course of a few days. BUT... it would be good to have a smarter controller. For example, the Midnite classic can be programmed to charge your batteries, and then when they are charged, to skip charging them for a time period that you specify.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: panel compatibility question
    mark68 wrote: »
    Thanks for the link. I saw on page 11 where it said AGM batteries can sit unused for a year or more unlike lead acid. Which I would never go that long. So I will shut down my system every time I leave from now on. Do you guys think I would be extending the life of the batteries by not cycling my batteries unless I am there using my system? I just saw that you have agm as well. Are you happy with them as opposed to lead acid?

    Please note AGM's are lead-acid batteries. They are just not flooded cells. They can sit longer with less (but not no) damage because they have a much lower self-discharge rate. They will sulphate, only slower and not to such an extreme because there is less sulphuric acid solution in an AGM.
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question
    vtmaps wrote: »
    Yes, a battery only has so many cycles in its life. Disconnecting them in your absence makes sense, but they should be fully charged when you leave. That may be a problem... are you going to run the generator so that they can get the 8 hours of absorb they need?

    With an undersized array, there is something to be said for leaving them connected to the controller while you are gone... they will get charged over the course of a few days. BUT... it would be good to have a smarter controller. For example, the Midnite classic can be programmed to charge your batteries, and then when they are charged, to skip charging them for a time period that you specify.

    --vtMaps

    I will leave the charge controller on to keep batteries charged while I am gone. I will turnoff the inverter and unplug my fridge when I leave. That was my thinking with the cycling. I should be extending their life by not charging/discharging when I am not there if I dont need to. Thanks for the info on the midnite classic. The ability to program when and how much charge the batteries get will help extend there life as well.
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question

    Well I got down and looked at my array. My 12 volt 125 watt panels are wired two together then to the combiner box that is set up with 15 amp breakers so my 4 panels are using two breakers. I have enough circuits in my combiner box to place ten panels on my array. So with this set up could I not just install 24 volt panels but instead of tieing two together run each panel straight to the combiner box? Would that not work as long as I don't exceed my 50amp charge controller I currently have. I am looking at a midnight classic down the road but want to add a few panels this year.
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question

    Just remember--All "24 volt" panels are not created equal.

    The typical "12 volt panel" designed for charging a 12 volt battery bank have Vmp in the range of ~17.5 to 18.6 volts or so... This is usually thought to be the "optimum" match of solar array voltage to lead acid charging voltage when used with a PWM charge controller. (as panels get hot in full sun, their Vmp-array falls--Can fall by upwards of 20% in very hot weather)

    So, a 24 volt panel designed to charge a 24 volt battery bank would have Vmp in the range of 35 to 38 volts or so (match two "12 volt panel" placed in series).

    However, there are many "GT Panels" (grid tied) designed for use with GT Inverter systems... The actual panel voltage does not matter as many panels are placed in series for efficient use with a GT solar inverter (10 panels or so in series).

    Many vendors call this "24 volt" panels--But they are really ~60 cell panels with Vmp~30 volts.

    That voltage is not high enough to charge a true 24 volt lead acid battery bank... And two panels in series is way to high of voltage to be used efficiently with a PWM charge controller (almost 1/2 of the array wattage is "lost" due to voltage miss-match).

    When using panels with a MPPT charge controller, the high voltage array can have advantages (lighter copper wire gauge between array and charge controller, better hot weather performance, etc.)... However, when matching panels from different vendors, you usually want to "match" the Vmp of the two panels (strings) within ~10% or better...

    A Vmp string of 30 volts vs another string of 35 volts is too far apart for the MPPT charge controller to get efficient power matching. It will work, but you simply will not get the optimum power because the array is not optimum and possibly because the charge controller will pick the "wrong" MPPT peak (i.e., could pick the 30 volt peak which takes current from both strings, or it could pick the 35 volt peak, and only pull current from the 35 volt string--As an example).

    If you have two "too different" panel strings, your only choice is to run two charge controllers. One for the original string and a second for different Vmp-array string (or sell the old panels and buy a complete set of new panels).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,503 ✭✭✭✭✭
    Re: panel compatibility question

    That should be fine, indeed if they are going to the same charge controller, and your installer understood this and suggested a separate combiner box, it might be time for a new installer. Just be sure to get 'true' 24 volt panels. Many sellers call 'grid connect' 24v panels that aren't designed for battery charging. Really panels designed for best connecting cells for the manufacturer, since the panels are typically run in long string to reach grid connecting voltages.

    Not sure it has been discussed, but you do understand the relationship between volts and amp when running batteries and panels in strings? When in a string(series connected) panels voltage adds and the amps run at the amp rating of the lowest panel. so if you have 2 - 12v panels, with a VMP of 17.5v and an Imp of 6amps, they act as a 35 volt 6 amp panel. So if you have 2 strings of 2 12v panels like this and 3 - 24v nominal panels at 36 VMP at 10amp Imp you have an array of 35v and 6+6+10+10+10 or 42 amps Imp, and of course typical running will be much less than this, though in the mountains of Colo. you may have ideal conditions more often.

    Batteries add in the same way
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question

    Thanks for the replies. My current panels are 17.9 vmpp each, 35.8 wired together. So I need a 24 volt panel that is within 10% of that to be compatible with my current panels. Might be one out there but the 24 volt panels I have seen are all near 30 vmpp. Outside of my parameters. So if I cannot find any panels that would work I will just keep going with 12 volt panels and that is fine. I was just trying to get up the most panel for the buck.
  • mark68mark68 Registered Users Posts: 17
    Re: panel compatibility question
    Photowhit wrote: »
    That should be fine, indeed if they are going to the same charge controller, and your installer understood this and suggested a separate combiner box, it might be time for a new installer. Just be sure to get 'true' 24 volt panels. Many sellers call 'grid connect' 24v panels that aren't designed for battery charging. Really panels designed for best connecting cells for the manufacturer, since the panels are typically run in long string to reach grid connecting voltages.

    Not sure it has been discussed, but you do understand the relationship between volts and amp when running batteries and panels in strings? When in a string(series connected) panels voltage adds and the amps run at the amp rating of the lowest panel. so if you have 2 - 12v panels, with a VMP of 17.5v and an Imp of 6amps, they act as a 35 volt 6 amp panel. So if you have 2 strings of 2 12v panels like this and 3 - 24v nominal panels at 36 VMP at 10amp Imp you have an array of 35v and 6+6+10+10+10 or 42 amps Imp, and of course typical running will be much less than this, though in the mountains of Colo. you may have ideal conditions more often.

    Batteries add in the same way
    My installer suggested a separate charge controller for the 24 volt panels I think for the variance in vmpp between the 12 and 24 volt panels. But did not say that specifically. He did not say anything about a separate combiner box. But I would still have to run new wire from my combiner box to my other charge controller. So if I cant find compatible 24 volt panels i will just fill out my current array with 12 volt panels . Do you think the energy I would lose from the vmpp exceeding 10% between the 12 and 24 volt panels would negate any benefit from installing the larger panels?
  • BB.BB. Super Moderators, Administrators Posts: 31,336 admin
    Re: panel compatibility question

    Vmp~30 volt panels will not (properly/efficienty) charge a 24 volt battery bank...

    A 24 volt battery bank needs >~29 volts to charge--And the Vmp in hot weather can fall by 10-20% (~30 volts * 0.8 = 24 volt Vmp). While the panels can charge (morning/evening & discharge battery bank, etc.)--It does not leave much room for wiring and controller voltage drops.

    With 30 volt Vmp panels, an MPPT charge controller with 2-3 30vmp panels in series for a "high voltage" array--that is a good solution.

    You can still use your 12 volt/true 24 volt array (Vmp>35 volts) with a (less expensive) PWM charge controller--And move your MPPT controller to the new non-standard battery Vmp voltage array.

    Move the combiner box over to the array too--And you will have plenty of room for expansion.

    No correct answers--Just options that can work fine. Do a couple paper designs for your ultimate system and see what would work best for you.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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